Nice idea, delete and create!
One can also use a { code block } that holds the input file open.
In this case one can even use the stdin (descriptor 0) instead of a descriptor 3.
It is more risky, since if anything goes wrong with writing to the new file, the old file still gets deleted.
Quote:
Originally Posted by disedorgue
under linux, the risk can be reduced
With "anything goes wrong" Scrutinizer perhaps meant the system crashing, the process being aborted for some kind of error or something such. In this cases you'd end up with a deleted file or a zero-length results file.
In my experience it pays off to deleted the source only once you are satisfied with the target and not one iota sooner.
With "anything goes wrong" Scrutinizer perhaps meant the system crashing, the process being aborted for some kind of error or something such. In this cases you'd end up with a deleted file or a zero-length results file.
Ok, but sed -i (or perl -i ) has the same problem, no ?
Ok, but sed -i (or perl -i ) has the same problem, no ?
No, they don't - but they have other problems which is why their usage is discouraged anyway:
What sed -i does is in fact opening a second file, write to it and then deleting the original and moving its temporary file where the original one was - BEHIND YOUR BACK!
The "advantage" is manifold: first you have no control over this. If something happens (crash, aborted task, ...) then the temporary file remains and clutters up space. That might happen to you with writing to a target file "manually" too, but in this case you are aware of the fact and might take cleaning actions. In case of sed -i you are probably not aware of this temporary file because you have been lied to by the program about its existence.
Second, even if you are aware of that: where would start searching for it? If i have some code like this:
Code:
if sed '<something>' /path/to/input > /some/output
mv /some/output > /path/to/input
else
print -u2 "Error executing sed ..."
fi
I have control over where the temporary file goes. And because of that i know where to look at to clean up. Do you know where sed -i puts its temporary file? i don't and the man page is busy telling you lies about the non-existence of the file, so it won't tell you either.
Third, and worst, IMHO: because of the mechanism you are most probably not aware that the files inode changes! This does not only mean the inode number but it may also mean that the ownership and/or the filemode may change (in case of a directory with SUID bits set). If one uses an explicit temporary file one is perhaps aware of this and does (from the example above:
Code:
if sed '<something>' /path/to/input > /some/output
mv /some/output > /path/to/input
chown someuser:somegroup /path/to/input
chmod ....
else
print -u2 "Error executing sed ..."
fi
But with the original file still in place (wink wink) why should one do that?
Finally, and perhaps most importantly: scripts should make as few assumptions about their surroundings as possible. Only GNU-sed understands the -i-option, but POSIX does not. Should i willingly limit my script to systems with a GNU-sed available? I don't think so.
All these reasons are why i strongly suggest not to use the -i option even if the sed in question supports it.
(GNU) sed -i and perl -i work differently as can be checked with strace.
GNU sed -i creates a new temporary file first and then moves it back to the original file if all is OK perl -i opens the file, unlinks it and then open a new file with the same name..
So GNU sed -i works in a safer way whereas perl -i does not..
Last edited by Scrutinizer; 10-14-2018 at 09:07 AM..
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10-12-2018
, but it is not much different from or offers advantages over simply creating a new file and then renaming it to the old file.